It is known that the physical properties of a material may be altered by exposing the material to electron (e-beam) radiation. In this respect, manufacturers treat certain products, such as plastic articles or animal feed, to produce a desired alteration in the products. In a conventional product irradiation process, a product is moved along a conveyor through an e-beam that scans across, i.e. traverses, the path of the conveyor. The e-beam is scanned across the conveyor by means of a scan horn that is a fixed distance from the surface of the conveyor. Depending upon the size of the product to be irradiated, the distance between the scan horn and the product may vary.
As the e-beam travels through air from the scan horn to the product, instabilities occur in the e-beam. These instabilities in the e-beam are the result of uncontrolled air plasma that is formed by the e-beam as a result of ionization of gases molecules, such as oxygen and nitrogen molecules in the air. Once the electron beams exit the accelerator through a foil window in the scan horn, the beam tends to increase in cross section, and the beam current density decreases. These events lead to two (2) negative effects in the irradiation process.
First, there is a decrease in the value of absorbed dose in the irradiated product.
Second, the angle of incidence of the scanning electron beam on the product decreases the penetration of the e-beam into the product. A 20% variation of absorbed dose may occur.
A still further problem with irradiating products in air is that certain products give off gases that accumulate at the surface of the product during irradiation. This accumulation of gases leads to uncontrolled variation of the parameters of the air plasma. The accumulation of gases may also lead to chemical reactions on the surface of a product being irradiated.
It is also known to use a high current e-beam from a pulsed electron accelerator to irradiate product(s). A high current beam from a pulsed electron accelerator (without a scanning system) exhibits the effects of the space charge in vacuum or air because of the high current and the high charge of the e-beam. For example, the diameter of an e-beam in vacuum for a beam current of 1 kA and a kinetic energy of 500 keV may increase 10 times over a distance of 12 cm. The propagation of this type of e-beam in air has the same problems with non-stability as described above. An additional problem is that the variation of current density tends to lead to a variation of dose distribution in an irradiated product.
The present invention overcomes these and other problems and provides a method and apparatus for the transport of an electron beam from the accelerator to a product to be irradiated.